The present invention relates generally to fastener-driving tools used for driving fasteners into workpieces, and specifically to combustion-powered fastener-driving tools, also referred to as combustion tools.
Combustion-powered tools are known in the art for use in driving fasteners into workpieces, and examples are described in commonly assigned patents to Nikolich U.S. Pat. Re. No. 32,452, and U.S. Pat. Nos. 4,522,162; 4,483,473; 4,483,474; 4,403,722; 5,197,646; 5,263,439 and 5,713,313, all of which are incorporated by reference herein. Similar combustion-powered nail and staple driving tools are available commercially from ITW-Paslode of Vernon Hills, Ill. under the IMPULSE® and PASLODE® brands.
Such tools incorporate a generally pistol-shaped tool housing enclosing a small internal combustion engine. The engine is powered by a canister of pressurized fuel gas, also called a fuel cell. A battery-powered electronic power distribution unit produces a spark for ignition, and a fan located in a combustion chamber provides for both an efficient combustion within the chamber, while facilitating processes ancillary to the combustion operation of the device. Such ancillary processes include: inserting the fuel into the combustion chamber; mixing the fuel and air within the chamber; and removing, or scavenging, combustion by-products. The engine includes a reciprocating piston with an elongated, rigid driver blade disposed within a single cylinder body.
A valve sleeve is axially reciprocable about the cylinder and, through a linkage, moves to close the combustion chamber when a work contact element at the end of the linkage is pressed against a workpiece. This pressing action also triggers a fuel-metering valve to introduce a specified volume of fuel into the closed combustion chamber.
Upon the pulling of a trigger switch, which causes the spark to ignite a charge of gas in the combustion chamber of the engine, the combined piston and driver blade is forced downward to impact a positioned fastener and drive it into the workpiece. The piston then returns to its original or pre-firing position, through differential gas pressures within the cylinder. Fasteners are fed magazine-style into the nosepiece, where they are held in a properly positioned orientation for receiving the impact of the driver blade.
The above-identified combustion tools incorporate a fan in the combustion chamber. This fan performs many functions, one of which is cooling. The fan performs cooling by drawing air though the tool between firing cycles. This fan is driven by power supplied by an onboard battery and, to prolong battery life, it is common practice to minimizing the run time of the motor. Also, short fan run time reduces fan motor wear (bearings and brushes), limits sound emitting from the tool due to air flow, and most importantly limits dirt infiltration into the tool. To manage fan ‘on time’, combustion tools typically incorporate a control program that limits fan ‘on time’ to 10 seconds or less.
Combustion tool applications that demand high cycle rates or require the tool to operate in elevated ambient temperatures often cause tool component temperatures to rise. This leads to a number of performance issues. The most common is an overheated condition that is evidenced by the tool firing but no fastener driven. This is often referred to as a “skip” or “blank fire.” As previously discussed, the vacuum return function of a piston is dependent on the rate of cooling of the residual combustion gases. As component temperatures rise, the differential temperature between the combustion gas and the engine walls is reduced. This increases the duration for the piston return cycle to such an extent that the user can open the combustion chamber before the piston has returned, even with a lockout mechanism installed. The result is the driver blade remains in the nosepiece of the tool and prevents advancement of the fasteners. Consequently, a subsequent firing event of the tool does not drive a fastener.
Another disadvantage of high tool operating temperature is that there are heat-related stresses on tool components. Among other things, battery life is reduced, and internal lubricating oil has been found to have reduced lubricating capacity with extended high temperature tool operation.
Thus, there is a need for a combustion-powered fastener-driving tool which reduces fan on time. In addition, there is a need for a combustion-powered fastener-driving tool which manages tool operating temperatures within accepted limits to prolong performance and maintain relatively fast piston return to pre-firing position.